(a) Field of the Invention
The present invention relates to a mold for hot stamping. More particularly, the present invention relates to a mold for hot stamping that enhances cooling efficiency and improves strength of a welding portion by improving a cooling method of the welding portion when stamping a heated TWB boron steel plate that is a blank for the hot stamping.
(b) Description of the Related Art
Generally, a vehicle body including a bumper beam of a vehicle is manufactured by pressing steel plates, and strength of the steel plates has a great influence on stability of the vehicle.
Recently, hot stamping has often been applied so as to meet high strength and light weight conditions of the steel plates. According to the hot stamping, a boron steel plate 111 is stamped in a hot state, as shown in FIG. 1.
The boron steel plate 111 is a steel plate containing a small amount of boron (element symbol “B”). In the boron steel plate 111, boron atoms are segregated at austenite grain boundaries at an appropriate temperature condition so as to lower free energy of the austenite grain boundaries. Therefore, generation of free ferrites is suppressed so as to greatly improve hardenability of steel (depth to which steel is hardened due to formation of martensite when quenching).
The hot stamping of the boron steel plate 111 will be briefly described.
The boron steel plate 111 with a ferrite crystal structure having tensile strength of about 500-800 MPa is heated up to 900° C. such that a boron steel plate 111 with an austenite crystal structure is formed, and the heated boron steel plate 111 with an austenite crystal structure is stamped in hot stamping molds 113 and 115. After that, the boron steel plate 111 with an austenite crystal structure is quickly cooled at a cooling speed of about 27° C./s-30° C./s such that a high-strength product with a martensite crystal structure having tensile strength of about 1300-1600 MPa is manufactured.
Such a hot stamping product 117 is four or five times as strong as and thinner than a product manufactured by another method using a steel plate. Therefore, weight may be reduced by up to 40% compared with a conventional product, and accordingly, a vehicle body having high strength but light weight can be manufactured.
The hot stamping can achieve high strength and light weight of the product, but manufacturing cost may be very high if the product is manufactured with only the boron steel plate 111.
Particularly, high strength is not needed at all parts but only at specific parts of the product used in the vehicle body.
Therefore, the hot stamping product has recently been manufactured by using a tailor welded blank (hereinafter called “TWB”) including the boron steel plate when manufacturing the vehicle body.
That is, a TWB 200 including the boron steel plate (hereinafter called “TWB boron steel plate”) has a part demanding high strength and another part. In this case, the one part is manufactured with the boron steel plate and the other part is manufactured with a ductile steel plate. The TWB boron steel plate 200 is manufactured by welding the boron steel plate and the ductile steel plate together. On the contrary, the TWB boron steel plate 200 may be manufactured with only boron steel plates, as shown in FIG. 2. In this case, the part demanding high strength is manufactured with a thick boron steel plate 111a and the other part is manufactured with a thin boron steel plate 111b. At this time, the TWB boron steel plate 200 is manufactured by welding the thick boron steel plate 111a and the thin boron steel plate 111b together.
Such a TWB boron steel plate 200 is also stamped once in the hot stamping molds 113 and 115 after being heated up to 900° C. by a heater. After that, the TWB boron steel plate 200 is quickly cooled (with a cooling speed of about 27° C./s-30° C./s) by an indirect cooling method due to heat conduction of the hot stamping molds 113 and 115 themselves such that a high-strength product is manufactured.
Herein, the indirect cooling method means a cooling method where the hot stamping molds 113 and 115 are cooled by a coolant circulating through coolant pathways formed in the hot stamping molds 113 and 115. That is, after a blank (i.e., TWB boron steel plate) at a high temperature is stamped in the molds, the coolant is supplied to the coolant pathways in the molds in a state in which the blank contacts the molds such that the blank is cooled by heat conduction.
According to the hot stamping, the temperature condition (higher than or equal to about 900° C.) is important but the cooling condition (27° C./s-30° C./s) is also important so as to secure strength of the manufactured boron steel plate 111.
If the TWB boron steel plate 200 manufactured with the boron steel plates 111a and 111b having different thicknesses is used as the hot stamping blank, a surface of a welding portion W of the TWB boron steel plate 200 is rough or has an irregular surface gradient due to a welding bead WB. Therefore, it is impossible to manufacture a forming surface of the mold so as to correctly match the welding portion W of all the blanks.
Since the welding portion W cannot completely contact the hot stamping molds 113 and 115, and particularly the upper mold 113, cooling efficiency of the welding portion W using heat conduction may be deteriorated.
As described above, it is hard to meet the cooling condition of the TWB boron steel plate 200 manufactured with the boron steel plates 111a and 111b having different thicknesses due to deterioration of cooling efficiency at the welding portion W after the hot stamping. Therefore, it is difficult to secure strength of the welding portion W.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.